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Scientists Implant Human Brain Cells in Mice

Just four short years ago, scientists first learned how to coax human embryonic stem cells to grow into a mass of brain cells called brain organoids. Like all embryonic research, the endeavors required the killing of human embryos. Now, secular science, undaunted by the destruction of human life, faces another ethical hurdle—research with the organoids is exploding, and some of the studies involve implanting human brain cells into rodents.

The clumps of cells are tiny, about the size of a lentil or an unborn baby at six weeks of gestation, but they pulse with the same kind of electrical energy that stimulates actual brains, they spawn new brain cells, and they develop the six layers of the cortex, the brain region that controls thought, speech, judgment, and other advanced functions, STAT News reported. Researchers hope doctors eventually will use the organoids to treat brain injury, stroke, schizophrenia, and autism.

But the whirlwind advances in these experiments place researchers in a quandary: Is it ethically right to implant human brain cells in an animal and, if so, how far should they be allowed to develop? It is entirely new ground, and “the science is advancing so rapidly, the ethics can't keep up,” said Christof Koch, president of the Allen Institute for Brain Science in Seattle.

Researchers have already conducted numerous studies involving human brain organoids, 21 of which were scheduled for presentation this week at the annual meeting of the Society for Neuroscience in Washington, D.C.

In one of the studies, Salk Institute researchers discovered that as the human organoids began to grow in mice, they connected to the rodents’ circulatory systems and developed mature brain cells that carried electrical signals to multiple regions of the rodents’ brains.

In another study Isaac Chen, a neurosurgeon at the University of Pennsylvania, implanted human organoids into the brain areas that process visual signals in 11 adult rats. The organoids survived for at least two months, Chen told STAT News, and showed “extensive” growth into the rats’ brains. When the scientists shined light on the rats’ eyes, cells in the implanted organoids fired, showing that the human brain tissue had not only integrated with the rats’ brains but was also functioning, he said.

The National Institutes of Health placed a moratorium on funding research that implants human stem cells into embryos or vertebrates, but there are no restrictions on implanting human organoids in animals.

“It brings up some pretty interesting questions about what allows us, ethically, to do research on mice in the first place—namely, that they’re not human,” Josephine Johnston, a bioeticist at The Hastings Center, told STAT News.

The Bible says God created each living thing “according to its kind” and humans in the image of God (Genesis 1). Brain function is one of the most obvious distinctions between humans and animals, but experiments with implanting human brain organoids in mice threaten to blur the boundary God created.